Wiring a detached garage from another detached garage?

Hey guys, I am needing some second opinions on my project. I have a detached garage(24x30) that is fed from a 100a breaker in the main which is 200a. I have built a second detached garage(30x42) that is 150ft. behind the first. I am wanting to put a 100a breaker in the first garages sub-panel and run 4/0 AL (4-4-4-6)SE to the second garage where i will have another subpanel. Where I have concerns is when running AL, can I run it 24ft. across my first garage before burying it in 2in. pvc. Or do I need to run CU across and then junction to AL before starting my run, and does either need to be in conduit while going overhead? If anyone could help me out on this it would be greatlly appreciated.

Are you running 4/0 SE cable or 4-4-4-6 cable? The latter is not large enough for a 100 amp feeder.

Sorry about the confusion, I was wanting to know which to run. 4/0 or 4-4-4-6, I thought 4awg was good up to 100a if using AL, and 2awg if using CU. Price wise I would rather use AL, which is only rated at 90deg.C, and can it be ran inside, my sub-panel is on the opposite wall so I have to go all the way across my garage before I exit. According to NEC this is not allowed, but I have been told that it is done everyday. So this is why I am on the fence.

You have your sizing mixed up. #2AL is good for 90 amps #3 Cu is good for 100 amps, and 4/0 Al is good for 180 amps. You need to do some load calculations because a your plan doesn't make sense.

Ok, this is obviouslly why I am asking for advice. So if someone could tell me what exactly I need to run from sub-panel to sub-panel that would be great!!!! Because I have been told that I can run SER which what I have found is mobile home ser. and its 4-4-4-6. BUT! NEC says it can only run from ser. to panel. I have been told that I can run it from panel to panel. So my question is what is everybody else using for a job like this?